Modern electronic circuits and systems use and process signals of increasing frequencies. For example, processors, such as microprocessors, microcontrollers, and digital signal processors, use relatively high clock frequencies. Other circuits and integrated circuits (ICs) commonly use high-frequency signals as reference or clock signals. Yet other circuits may seek to increase the frequency of a given signal by, for example, doubling its frequency.
Because of the relatively high frequencies involved, however, circuit board traces, connectors, wires, sockets, IC pads and bonding wires, etc., may impose unacceptably high parasitic impedances in the path of high-frequency signals. As a result, designers often provide high-frequency signals, such as clock signals, on-chip or locally, as needed, by using a reference signal of a lower frequency.
FIGS. 1A and 1B show two conventional ways of doubling a clock frequency. FIG. 1A illustrates a circuit 10 for doubling a clock frequency by using a phase-locked loop (PLL), whereas circuit 50 in FIG. 1B uses a delay-locked loop (DLL). The details of the circuitry and operation of the circuits in FIGS. 1A and 1B fall within the knowledge of persons of ordinary skill in the art.